Re: better wake-balancing: respin

[Ingo Molnar <mingo@elte.hu>]

* Nick Piggin <nickpiggin@yahoo.com.au> wrote:

> Ingo, I wasn't aware that tasks are bouncing around wildly; does your 
> patch improve things? Then by definition it must penalise workloads 
> where the pairings are more predictable?

for TPC, most of the non-to-idle migrations are 'wrong'. So basically 
any change that gets rid of extra migrations is a win. This does not 
mean that it is all bouncing madly.

> I would prefer to try fixing wake balancing before giving up and 
> turning it off for busy CPUs.

agreed, and that was my suggestion: improve the heuristics to not hurt 
workloads where there is no natural pairing.

one possible way would be to do a task_hot() check in the passive 
balancing code, and only migrate the task when it's been inactive for a 
long time: that should be the case for most TPC wakeups. (This assumes 
an accurate cache-hot estimator, for which another patch exists.)

> Without any form of wake balancing, then a multiprocessor system will 
> tend to have a completely random distribution of tasks over CPUs over 
> time. I prefer to add a driver so it is not completely random for 
> amenable workloads.

but my patch does not do 'no form of wake balancing'. It will do 
non-load-related wake balancing if the target CPU is idle. Arguably, 
that can easily be 'never' under common workloads.

	Ingo